Olivier Drapier, "The Higgs Boson, Cornerstone of the Theory of Matter"

Discovered after forty years of research, the Higgs boson is "the" elementary particle that we needed to understand the origin of matter. The physicist Olivier Drapier [1]. explains what is involved in this major scientific breakthrough.

Huma: On 4 July, the discovery of the Higgs boson was echoed in all the media. Why is the proof of the existence of this elementary particle so important?

Olivier Drapier: To understand what is involved in this major discovery, we must first understand the theory that describes matter: the standard model. According to this model, matter is composed of two types of particles: quarks, which constitute protons and neutrons, which make up atomic nuclei, and leptons, such as electrons or neutrinos. Four forces permit these particles to interact or to transform: gravitation, which structures the universe on a large scale, the electromagnetic interaction, which keeps the electrons around atomic nuclei, the strong interaction, which glue the quarks in protons and neutrons and ensures the cohesion of nuclei, and finally, the weak interaction, which causes some particles to transform into others, as is the case in certain phenomena of radioactivity. In this model, the electromagnetic and weak interactions should be the same, and the particles should not have mass. Mass is inertia, the ability to resist a change in velocity [2]. However, this is contrary to our experience. The electromagnetic interaction propagates to infinity, while the weak interaction acts only over distances much smaller than the size of the proton. In addition, elementary particles have very different masses, from very light neutrinos, up to quarks, at least a hundred billion times heavier. The "electroweak symmetry breaking", proposed among others by Peter Higgs in 1964, overcomes this difficulty: there is in the universe a field with which particles interact to a greater or lesser degree, giving them a greater or lesser mass. By the same mechanism, the particles that carry the weak interaction (called W and Z bosons) acquire mass, while those that transmit the electromagnetic interaction (photons) do not, which explains why these two forces are so different. If this field exists, then there is a particle, the Higgs boson, which is the excitation of this field, and which one can cause to appear in certain circumstances. It’s rather the keystone of the standard model.

Huma: It took forty years to discover the Higgs boson, why so long?

Olivier Drapier: Firstly, it remains to verify that the new boson discovered at CERN really has all the properties expected of the Higgs boson, and this can take a lot of time. The difficulty in discovering a new particle is that you have to create it. This you do using Einstein’s equation E = mc ², which says that mass is a form of energy. We build particle accelerators to cause collisions between particles. Part of the impact energy is transformed into other particles. Since the Higgs boson is very heavy, it takes a lot of high-energy shocks in order to create one such particle from time to time. It is only detectable by observing the elementary particles into which it is instantaneously transformed. So we need particle detectors, which are like giant cameras, several tens of meters in size, and up to 12500 tons in weight, thus with half the mass of the Eiffel Tower!, devices at the forefront of technology, in order to see them. Ultimately, this discovery has been made possible thanks to technological advances in recent years, but this, we had no way of knowing in advance.

Huma: What has motivated researchers during all these years?

Olivier Drapier: The problem in science is not to test theories, but to rule them out. You can never know whether a theory is right, because you will never test all its predictions without exception. On the other hand, if it is false, just find one example that disproves it, and it’s garbage. The Higgs model predicts that there is at least one boson that corresponds to the Higgs field. If you find nothing, then the theory is ready to be thrown away.

Huma: Just so, does basic research still have the means to continue? Why is this essential? And what is the interest for society in funding this type of research?

Olivier Drapier: In France, the tangle of sources of project financing, such as the ANR [3] implemented by previous governments, is an obstacle for us. This is undeniable. In the current economic climate, it may seem futile to invest in science, where the results obtained will not have practical utility for decades. But what has not been discovered can not be used. It is not through seeking to improve the candle that electricity was invented. Einstein’s relativity, for example, is applied daily in GPS systems, which otherwise would not work. I’m sure Einstein would have laughed in your face if you had dared predict it. And humans are like that, they want to understand. This is what distinguishes us from other animals. What is more, particle physics pushes industry to create ever more innovations in high-tech fields. For example, superconductors, grid computing, the Web (invented at CERN) ... Finally, this science as a whole provides employment for thousands of scientists from all over the country, sometimes in spite of conflicts between nations, with only peaceful purposes, and not mercantile. It seems to me that this is a just reason to have faith in humanity.

Huma: After the LHC [4], what is the next step toward a better understanding of matter? Will we need a more powerful particle accelerator?

Olivier Drapier: From here, the project is to exploit the LHC for at least fifteen years! We are only at the beginning. The mass of the Higgs boson suggests that there may be new physics beyond the standard model. Perhaps new particles predicted by supersymmetry theories? The answer in a few years. In parallel, we must build another type of machine, an accelerator of electrons that will permit precision measurements. This accelerator will be linear, unlike the LHC (which is a circular tunnel 27 km in circumference) and may be built in Japan. To open Nature’s safe of secrets, the electron accelerator is a bit like a stethoscope, while the LHC it is rather like dynamite ...

Olivier Drapier is Director of Research at CNRS, laboratory Leprince-Ringuet, a joint research unit of Ecole Polytechnique and CNRS/IN2P3.

[1] Olivier Drapier is Director of Research at CNRS , laboratory Leprince-Ringuet, joint research unit of Ecole Polytechnique and CNRS/IN2P3.

[3] The Agence nationale de la recherche (ANR) is a French government agency that finances research, particularly research of a collaborative nature between universities and industries, and is classed as "a public establishment of administrative character". It provides direct financing to both public and private research groups, via contracts of limited duration.